A preferred application for the present invention is in high voltage alternating current (AC) three phase circuit breakers and reclosers, the latter being a type of circuit breaker. Therefore, the background of the invention is described below in connection with such devices. However, it should be noted that, except where they are expressly so limited, the claims at the end of this specification are not intended to be limited to applications of the invention in a high voltage three phase AC circuit breaker.
A high voltage circuit breaker is a device used in the distribution of three phase electrical energy. When a sensor or protective relay detects a fault or other system disturbance on the protected circuit, the circuit breaker operates to physically separate current-carrying contacts in each of the three phases by opening the circuit to prevent the continued flow of current. A recloser differs from a circuit breaker in that a circuit breaker opens a circuit and maintains the circuit in the open position indefinitely, whereas a recloser may automatically open and reclose the circuit several times in quick succession to allow a temporary fault to clear and thus, avoid taking the circuit out of service unnecessarily.
The major components of a circuit breaker or recloser include the interrupters, which function to open and close one or more sets of current carrying contacts housed therein; the operating or driving mechanism, which provides the energy necessary to open or close the contacts; the arcing control mechanism and interrupting media, which create an open condition in the protected circuit; one or more tanks for housing the interrupters; and the bushings, which carry the high voltage electrical energy from the protected circuit into and out of the tank(s). In addition, a mechanical linkage connects the interrupters and the operating mechanism.
Circuit breakers may differ in the overall configuration of these components. However, the operation of most circuit breakers is substantially the same regardless of their configurations. For example, a circuit breaker may include a single tank assembly which houses all of the interrupters. U.S. Pat. No. 4,442,329, Apr. 10, 1984, "Dead Tank Housing for High Voltage Circuit Breaker Employing Puffer Interrupters," discloses an example of the single tank configuration. Alternatively, a separate tank for each interrupter may be provided in a multiple tank configuration. An example of a multiple tank configuration is depicted in FIG. 1.
As shown in FIG. 1, a prior art circuit breaker assembly 1 includes three cylindrical metal tanks 3. The three cylindrical tanks 3 form a common tank assembly 4 which is preferably filled with an inert, electrically insulating gas such as SF.sub.6. The tank assembly 4 shown in FIG. 1 is referred to as a "dead tank" in that it is at ground potential. Each tank 3 houses an interrupter (not shown in FIG. 1). The operation of the interrupter is described below. An interrupters are provided with terminals which are connected to respective spaced bushing insulators. The bushing insulators are shown as bushing insulators 5a and 6a for the first phase; 5b and 6b for the second phase; and 5c and 6c for the third phase. Associated with each pole or phase is a current transformer 7. In high voltage circuit breakers, the pairs of bushings for each phase are often mounted so that their ends have a greater spacing than their bases to avoid breakdown between the exposed conductive ends of the bushings. In lower voltage applications, such spacing may not be required. The operating mechanism that provides the necessary operating forces for opening and closing the interrupter contacts is contained within an operating mechanism housing 9. The operating mechanism is mechanically coupled to each of the interrupters via a linkage 8.
A cross section of an interrupter 10 is shown in FIGS. 2A-C. The interrupter provides two sets of contacts, the arcing contacts 12 and 14 and the main contacts 15 and 19. Arcing contacts 12 and main contacts 19 are movable, as described in more detail below, to either close the circuit with respective contacts 14 and 15 or to open the circuit. FIG. 2A shows a cross sectional view of the interrupter with its contacts closed, whereas FIG. 2C shows a cross section of the interrupter with the contacts open.
The arcing contacts 12 and 19 of high voltage circuit breaker interrupters are subject to arcing or corona discharge when they are opened or closed, respectively. As shown in FIG. 2B, an arc 16 is formed between arcing contacts 12 and 14 as they are moved apart. Such arcing can cause the contacts to erode and perhaps to disintegrate over time. Therefore, a known practice (used in a "puffer" interrupter) is to fill a cavity of the interrupter with an inert, electrically insulating gas that quenches the arc 16. As shown in FIG. 2B, the gas is compressed by piston 17 and a jet or nozzle 18 is positioned so that, at the proper moment, a blast of the compressed gas is directed toward the location of the arc in order to extinguish it. Once an arc has formed, it is extremely difficult to extinguish it until the arc current is substantially reduced. Once the arc is extinguished as shown in FIG. 2C, the protected circuit is opened thereby preventing current flow.
Typically a bank of shunt capacitors is coupled between the arcing contacts to control the arcing by equalizing the voltages at the respective breaks in a multi-interrupting point type circuit breaker, i.e., one with more than one set of contacts. A capacitor coupled between contacts may also be used in a single-break circuit breaker. The bank of shunt capacitors is typically arranged within a dead tank to surround an arc-extinguishing chamber therein. It is further known to control arcing utilizing pre-insertion or closing resistors, as disclosed in U.S. Pat. No. 5,245,145, Sep. 14, 1993, "Modular Closing Resistor"(assigned to ABB Power T&D Company Inc.).
Voltage and current transients generated during the energization of shunt capacitor banks have become an increasing concern for the electric utility industry in terms of power quality for voltage-sensitive loads and excessive stresses on power system equipment. For example, modern digital equipment requires a stable source of power. Moreover, computers, microwave ovens and other electronic appliances are prone to failures resulting from such transients. Even minor transients can cause the power waveform to skew, rendering these electrical devices inoperative. Therefore, utilities have set objectives to reduce the occurrence of transients and to provide a stable power waveform.
Conventional solutions for reducing the transients resulting from shunt capacitor energization include circuit breaker pre-insertion devices, for example, resistors or inductors, and fixed devices such as current limiting reactors. While these solutions provide varying degrees of mitigation for capacitor bank energization transients, they result in added equipment, added cost, and can result in added reliability concerns.
The maximum shunt capacitor bank energization transients are associated with closing the circuit breaker at the peak of the system voltage waveform, i.e., where the greatest difference exists between the bus voltage, which will be at its maximum, and the capacitor bank voltage, which will be at a zero level. Where the closings are not synchronized with respect to the system voltage, the probability for obtaining the maximum energization transients is high. One solution to this problem is to add timing accuracy to synchronously close the circuit breaker at the instant the system voltage is substantially zero. In this way, the voltages on both sides of the circuit breaker at the instant of closure would be nearly equal, allowing for an effectively "transient-free" energization.
While the concept of synchronous or zero-crossing closing is a simple one, a cost-effective solution has been difficult to achieve, primarily due to the high cost of providing the required timing accuracy in a mechanical system. U.S. Pat. No. 4,306,263, Dec. 15, 1981, entitled "Synchronous Closing System and Latch Therefor," discloses a synchronous closing system wherein the circuit breaker main contacts close within about 1 millisecond of a zero crossing by inhibiting the hydraulic pressure utilized to close the interrupter contacts using a latch controlled mechanism. However, this synchronous closing system is incapable of providing synchronization for each phase or pole individually. Thus, while one phase may be closed synchronously, avoiding transients in that phase of the circuit, harmful transients may be produced by closing the contacts in one or both of the other phases.
One solution might be to utilize three separate operating mechanisms and corresponding linkages to synchronously control the operation of each pole individually. U.S. Pat. No. 4,417,111, Nov. 22, 1983, entitled "Three-Phase Combined Type Circuit Breaker," discloses a circuit breaker having a separate operating mechanism and associated linkage for each of the three phases or poles. However the use of three separate operating mechanisms and associated linkages is expensive and increases the overall size and complexity of the circuit breaker.
U.S. Pat. No. 4,814,560, Mar. 21, 1989, "High Voltage Circuit Breaker" (assigned to Asea Brown Boveri AB, Vasteras, Sweden) discloses a device for synchronously closing and opening a three-phase high voltage circuit breaker so that a time shift between the instants of contact in the different phases can be brought about mechanically by a suitable choice of arms and links in the mechanical linkage. This linkage uses an a priori knowledge of the time required to close and open the interrupter contacts in each of the three phases. The time differences can be accounted for by an appropriate design of the mechanical linkage. However, such a linkage cannot support dynamic monitoring of the zero-crossings for each phase to achieve independent synchronization. Moreover, the mechanical linkage disclosed would require mechanical adjustments over time to account for variations in the circuit breaker performance and operating conditions which often change over time.
A dependent pole operating mechanism has been used in circuit breakers to generate the initial driving forces required to open and close the interrupter contacts. Dependent pole operation refers to the limited capability of the operating mechanism to close or open all three phases of the circuit simultaneously. A prior art example of a dependent pole mechanism and mechanical linkage implemented in a three-phase circuit breaker is shown in FIG. 3. As shown in FIG. 3, operating mechanism 20 provides a single connecting rod 22. Connecting rod 22 is interfaced with linking element 26 via lever 24. Linking elements 25 and 26 preferably form a single linking shaft linking together the terminal portions of each of the three interrupters (not shown in FIG. 3). In operation, the connecting rod 22 is driven up or down thereby pivoting lever 24. As lever 24 pivots, the linking elements 25 and 26 rotate. The linking elements are preferably coupled to bell cranks provided in the terminal portion of the interrupters (not shown in FIG. 3) which pivot in response to the rotation of the linking elements to open and close the contacts of the interrupters. It should be understood that each of the interrupters housed in tanks 3 will open and close simultaneously in response to the movement of connecting rod 22.
Recently an independent pole operating mechanism has been developed which provides an individually controlled driving force for opening and closing each phase of the circuit breaker independently. By utilizing the independent pole operating mechanism, each phase can be dynamically and synchronously switched individually. Thus there is a need to provide a mechanical linkage to operate effectively with the independent pole operating mechanism. To eliminate the necessity of redesigning the entire circuit breaker to implement the new independent pole operating mechanism, it is desirable to cost-effectively adapt existing circuit breaker linkages, such as linkage 8 shown in FIG. 1. Moreover, the mechanical linkage for use with the independent pole operating mechanism should not increase the size of the circuit breaker, or require complex assembly or maintenance steps to ensure that the circuit breaker functions properly.